Nonwoven webs find application in a number of end uses, including paper towels, disposable diapers, filtration products, disposable wipes, and the like. Nonwoven products or fabrics comprise loosely assembled webs or masses of fibers bound together with an adhesive, polymer or polymeric binder. Fibers often are comprised of cellulosic or polymeric materials such as polyesters, polyamides, polyacrylates, and the like. The base web of nonwoven fibers, to which the polymeric binder is applied, can be produced by carding, garnetting, airlaying, papermaking procedures, or other known operations. Bonding fibers in place with a polymeric binder is much less expensive than the use of conventional spinning and weaving to form woven fabrics and the bonded nonwoven fabrics can be made in a much greater range of thicknesses per unit weight, with more homogeneous structures, no unraveling tendency, and with greater water absorbency, porosity, and resiliency.
Two major factors that lead to acceptable nonwoven products are the wet tensile strength and “feel” of the nonwoven product. Personal care products such as tissues, handwipes and sanitary napkins must have sufficient wet tensile strength to remain intact when wet and sufficient softness or feel when in contact with the skin. However, to achieve desirable or sufficient wet tensile strength it has been common practice to elevate the dry tensile strength of the polymer or use higher add-on levels of polymeric binder. Higher dry tensile strengths in a nonwoven product tend to impart stiffness or hardness to the product and make the nonwoven product uncomfortable to the touch. Higher add-on levels of polymeric binder are therefore undesirable because of the “feel” of the product and the cost of using larger amounts of polymeric binder.
There has been an industry move to improve the wet tensile strength of a nonwoven product in relation to its dry tensile strength with the dry tensile strength of a nonwoven product generally being secondary to its wet tensile strength. A product that has a high wet/dry tensile strength ratio is desired because it generally permits a lower add-on level of polymer to the nonwoven product, thereby improving the “feel” of the product and reducing manufacturing costs without sacrificing wet strength.
Polymeric binders based upon a vinyl acetate and ethylene backbone incorporating a self crosslinking monomer have been widely used in the nonwoven industry. Ethylene in the polymer provides softness to the product and is low cost. But, softness in the product often comes at the expense of wet tensile strength. Increasing the level of self crosslinkable monomer in the polymer often is not a viable option to increased wet strength. Against that background, the nonwoven industry is desirous of self-crosslinking vinyl acetate-ethylene based polymers having increased wet/dry tensile strength ratios at conventional add-on levels.
Representative of various vinyl acetate based binder compositions used for producing nonwoven products include:
U.S. Pat. No. 3,081,197 (Adelman, 1963) discloses a nonwoven binder comprising polymers of vinyl acetate, a polymerizable compound suited as an internal plasticizer, e.g., butyl acrylate and dibutyl maleate, and a post-curable comonomer such as N-methylol acrylamide. The binders can be prepared by interpolymerization of the monomers in aqueous dispersion systems containing nonionic, anionic or cationic dispersing agents.
U.S. Pat. No. 6,562,892 (Eknoian et al., 2003) discloses ethylene-vinyl acetate emulsion polymers having greater than 55 percent by weight ethylene which is water dispersible but non-dispersible in an aqueous solution containing 0.5 percent or more of an inorganic salt and is stabilized with a hydrophilic polymeric colloid. The hydrophilic polymeric colloid contains at least one hydrophilic monomer. The hydrophilic monomer may be an acidic monomer containing a carboxylic acid, e.g., a carboxylic acid or dicarboxylic acid, a sulfonic acid, or phosphonic group.
U.S. Pat. No. 3,137,589 (Mannheim et al., 1964) discloses binders comprising a copolymer of an alpha, beta-unsaturated carboxylic acid amide substituted on the nitrogen by at least one methylol group and another unsaturated polymerizable compound.
U.S. Pat. No. 3,380,851 (Lindemann et al., 1968) discloses a binder comprising an interpolymer of vinyl acetate-ethylene-N-methylol acrylamide. The ethylene content is from 5 to 40% by weight.
U.S. Pat. No. 4,449,978 (Iacoviello, 1984) discloses a nonwoven product formed from a nonwoven web of fibers bonded together with a binder comprising an interpolymer of vinyl acetate, ethylene, N-methylol acrylamide, and acrylamide monomers. The nonwoven products are reported to have low residual free formaldehyde content and good tensile properties.
U.S. Pat. No. 5,540,987 (Mudge et al., 1996) discloses the formation of formaldehyde free and formaldehyde reduced vinyl acetate-ethylene binders for nonwoven products. These binders are formed by emulsion polymerization using an initiator system based upon an organic peroxide and ascorbic acid. The crosslinking agent can be N-methylolacryl amide for nonwovens of reduced formaldehyde and N-iso-butoxy methyl acrylamide for formaldehyde free nonwovens.
US 2003/0232559 A1 (Goldstein et al., 2003) discloses adhesive polymeric binders comprised of a polymer containing polymerized units of vinyl acetate, ethylene, vinyl chloride, and a self-crosslinking monomer.